This invention relates to the fields of image processing and projection. More particularly, a computer-controlled projection system and methods of using the system to project an image without keystone distortion are provided.
An image projector or projection system is often used to produce an enlarged version of a video image. For example, an original image may be rendered on an LCD (Liquid Crystal Display) or other device through which light may be projected onto a viewing surface, in order to magnify or enlarge the original image. Where the projection system is aligned so as to project an image perpendicularly upon the screen, the projected image may accurately reproduce the proportions of the original image. However, projection systems are often situated at an angle to the viewing screen. As a result, the projected image typically suffers from what is known as keystone distortion and thus may differ substantially from the original image.
In particular, when the projection axis of the system is not perpendicular to the viewing screen, a portion of the projected image may appear distorted (e.g., stretched or compressed). For example, if the projector is situated below a line normal to the viewing area of the screen, then the image must be projected upward to the screen. As a result, the upper portion of the image may appear expanded or stretched in relation to the bottom portion, or the bottom portion of the image may appear condensed or contracted compared to the upper portion. The projected image may thus take on a trapezoidal or keystone shape. Similarly, if the projector is situated above or to one side or the other of a line normal to the viewing screen a different portion of the projected image may appear distorted. Depending upon the resolution or detail of the image being projected, the distortion may significantly degrade one""s appreciation of the image.
A projector may be situated at a non-perpendicular angle to a viewing screen because of transient or temporary use of the equipment, because of a lack of space in front of the screen, because the projector may obstruct a person""s view if placed normal to the center of the screen, and for other reasons. As the projector is moved further from the screen, a larger projected image may be possible but the keystone distortion increases commensurately.
One method by which the keystone distortion problem has been addressed involves mechanical compensation. In such a method, an LCD panel on which the original image is rendered, and through which light is passed to yield the projected image, may be rotated so as to be situated parallel to the plane in which the screen is located. This solution may cause high cost, however, and require manual adjustment for different environments. Each time the viewing screen or projector changes position, an LCD panel may need to be mechanically adjusted. This type of correction thus takes time to effect and depends upon the manual dexterity of an operator.
Beyond the keystone distortion problem, some projection systems incorporate multiple LCD panels (e.g., one for each of the three primary colors) in order to improve image quality. In such a case, the complexity of the system increases accordingly and further complicates correction of keystone distortion. In addition, present LCD projector systems lack the ability to easily impart useful image modifications such as luminance equalization or variably sized images.
In one embodiment of the invention a system and methods are provided for modifying an image for projection without keystone distortion, where the projection system is aligned at a non-perpendicular angle to the surface on which the image is projected. In this embodiment an original image is resized, if necessary, and reshaped to offset keystone distortion that may otherwise occur during projection of the image. In particular, an original image may be electronically deformed or altered for rendering on one or more LCD panels within a projector or projection system. The deformed image is configured such that when projected upon a viewing screen, the original image is recreated with the benefit of magnification or enlargement and retention of the original image""s proportions, but without significant keystone distortion.
In this embodiment of the invention a vertical offset of the projection apparatus (e.g., the apparatus is rotated vertically about a horizontal axis) results in tilt angle xcex8 between the projection axis of the apparatus and a line normal to the viewing screen. In this case images are projected upward or downward to the viewing screen. Similarly, pan angle xcex1 a results from a horizontal offset of the apparatus (e.g., the apparatus is rotated horizontally about a vertical axis). When either or both tilt angle xcex8 and pan angle xcex1 are not equal to zero, the necessary deformation operations may be performed separately or in combination.
In one embodiment of the invention an apparatus for preventing keystone distortion comprises a digitizer module in which an original image is resized (e.g., decimated or reduced) for storage in memory and a generator module in which the stored memory is resized (e.g., enlarged or expanded) for rendering on a display device (e.g., an LCD panel). Deformation of an image to offset or prevent keystone distortion may be performed in either or both of these modules. If both tilt angle xcex8 and pan angle xcex1 are non-zero, the image may be deformed for one angle and then rotated (e.g., ninety degrees) and deformed to offset the other.
The extent and effect of the deformation performed to prevent or offset keystone distortion in an embodiment of the invention may depend on several factors. These factors may include the magnitude of tilt angle xcex8 or pan angle xcex1, the distance from the projection light source to the display device (e.g., an LCD panel), the dimensions of the original image, the dimensions of the usable area of the display device, etc.
An original image may be deformed on a line-by-line, pixel-by-pixel or other basis. In one embodiment of the invention several parameters are calculated to identify the usable area of the display device, the number of lines in an image to be rendered and projected, the number of positions (e.g., pixels) in each line, etc. Then, for each selected or usable portion (e.g., pixel) of the display device, a corresponding portion of the original image (e.g., a pixel or set of pixels) is identified by applying one or more geometrical or mathematical relationships. The characteristics (e.g., color, intensity) of the corresponding portion of the original image may then be applied to the selected portion of the display device. As a result of deforming an original image and copying its characteristics, the image that is rendered on a display device and projected onto a viewing screen may reproduce the proportions of the original image (on a magnified scale) without significant or appreciable keystone distortion.
In one alternative embodiment of the invention a method and apparatus are also provided for ensuring uniform illumination intensity of the projected image. In particular, the intensity of an image rendered on a display device for projection purposes may be attenuated or otherwise adjusted in order to equalize the image""s intensity.